Signal equalization is the reversal of distortion incurred by a signal transmitted through an imperfect channel. Equalization makes the frequency response of a channel flat (i.e., no distortion) across the bandwidth under consideration. In an equalization operation, the frequency domain attributes of the desired signal at the input of the channel are reproduced at the output of the channel. Equalization is widespread, for example, radars, telephones, DSL lines, and television cables use equalizers to prepare data signals for transmission.
An equalizer circuit generates an output signal by equalizing its input signal. A typical equalizer circuit includes at least two interconnected and mutually interfering equalizers that exhibit different center frequencies. The gains of the two mutually interfering equalizers at the respective center frequencies are controllable by external control signals. The input signal of the equalizer circuit is spectrally weighted based on the external control signals. In digital signal processing, an equalizer circuit reduces intermodulation interference to allow recovery of the transmitted signal, typically by a simple linear filter or a complex algorithm.
A linear equalizer applies the inverse of the channel frequency response to the received signal to restore the signal after it leaves the channel. Although linear equalizers are simple to construct, most practical equalizers are made based on non-linear equalizing techniques due to the effects introduced due to time varying signals. Linearity is typically a difficult requirement in radio frequency (RF) systems. Nonlinear equalization (NLEQ) algorithms have been shown to substantially reduce spurs, but only if the NLEQ is performed on every individual channel. This creates a substantial amount of I/O and real time processing that is preferred to be mitigated and moved to the backend processing.
FIG. 1 is a simplified block diagram of a transfer function for a conventional non-linear equalization. As shown, there are N parallel channels X[1] to X[N], each with unique non-linear characteristics in this example. One of a plurality of independent non-linear transfer functions, F1, . . . FN, in each respective channel 1 to N to represent the unique non-linear frequency response of the respective channel. Additionally, one of a plurality of independent post-distortion filter G1−1, GN−1, is also placed in each channel to corresponding unique non-linear distortion so that the corrected signal with a flat frequency response results in that channel. The output of each post-distortion filter G1−1, GN−1 is added together by a summer Σ to provide a distortion free output signal Y[n]. As discussed above, this conventional approach requires a substantial amount of I/O and real time processing since a post-distortion filter G−1 is required for each channel.